When a massive star explodes as a supernova, its outer layers blast into space at millions of miles an hour. But the dying star's heart collapses into a tiny neutron star or black hole. Astronomers recently found what may be the closest such stellar remnant to Earth -- and it's located in the most familiar star pattern in the sky.

A neutron star is just a few miles across, but it weighs more than the Sun. As a result, a neutron star is incredibly dense, and its surface gravity is extremely powerful: Drop a pebble just four feet above a neutron star, and it would slam into the star's surface at five million miles per hour.

A black hole is even more extreme. It's a collapsed star, also born in a supernova explosion, with such strong gravity that nothing can escape from it -- not even light, the fastest thing in the universe.

Astronomers examined a faint star that's just south of Alioth, a bright star in the handle of the Big Dipper. They discovered that the faint star moves back and forth a bit, indicating that it's tugged by another star. This other star is at least 40 percent more massive than the Sun.

A normal star this massive would shine brightly. But no one has ever seen this star, so it must be a black hole or a neutron star. Whatever it is, it's just 160 light-years from Earth. That makes it a record breaker -- the closest known remnant of a supernova.